To change the MTU of an interface on GNU/Linux, you just need to use ifconfig command to do so, like this for example
sudo ifconfig eth0 mtu 1492
To change it permanently on Debian, put it in the /etc/network/interfaces file, where almost all network parameters are found. To do this, just add a line mtu to the definition of your interface and save the file.
sudo gedit /etc/network/interfaces
iface eth0 inet static address 192.168.0.1 network 192.168.0.0 gateway 192.168.0.254 netmask 255.255.255.0 mtu 1492
For dialup users: the Maximum Transmission Unit (MTU) value can be changed within the file